add SkSL support for mix, cross, and dot

Bug: skia:
Change-Id: Ic6c16f363c5f2aedaf1aee9b3b8ee23e37ea24e4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/215448
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
6 files changed